Friction devices such as clutches or brakes, especially for automotive vehicle applications, e.g. for the shifting of a multispeed transmission under load, generally comprise two relatively movable members, one of which is hydraulically displaceable, (i.e. is a piston or is coupled to a piston) which are urged toward one another (hydraulically or by spring force) to bring the two movable members or parts coupled therewith into frictional engagement.
This principle applies to single disk, multiplate and other clutches and brakes.
Before full engagement of the friction device, the movable member has, therefore, a certain axial displacement and, upon such engagement, there is practically no further relative movement between the two members, i.e. the two members are coupled for full torque transmission between them.
Friction devices of this type may have a controlled engagement, i.e. until there is full mutual coupling of the two members, it may be desirable to apply the axial pressure in a controlled manner so as to operate under predetermined friction characteristics, permit a certain amount of controlled slip, or prevent sudden coupling of the two members.
Friction devices of this type, moreover, are subject to wear, i.e. linings, friction surfaces and the like tend to reduce in thickness with use so that the displacement of the movable member until initial engagement and full engagement of the clutch or brake will vary in a progressive manner.
Friction clutches and brakes of the type with which the present invention is concerned may be any torque-transmitting device of the type described, whether intended to couple two shafts together or to brake rotation of a shaft.
For example, they may be used with considerable effectiveness in multispeed transmissions which are shiftable under load having a number of planetary gear sets which can be coupled together, to various shafts or partly immobilized utilizing hydraulically operated clutches and brakes with controlled coupling of the respective members so that, for example, the load need never be fully disconnected from the drive. Such transmissions are therefore shiftable under load.
Friction devices such as clutches and brakes have, in the past, been equipped with hydromechanical devices or elements which responded to the displacement of the movable member, e.g. to compensate for wear. However, these devices have not been found to be fully satisfactory when the controlled application of the friction pressure is desirable between the point of initial frictional interengagement and full frictional coupling. In addition, the mechanical or hydraulic adjusting systems are relatively complex, expensive, prone to wear and unreliable unless effective maintenance procedures are utilized.